The invention relates generally to improvements in battery chargers. In particular, the invention relates to a system and method for charging a lead acid battery below the battery gassing potential, desulfating deeply discharged lead acid batteries, and analyzing the quality of a battery being charged.
It is typically undesirable to charge a sealed battery, also called Valve Regulated Lead Acid (VRLA) battery, above its electrolyte gassing potential, or simply xe2x80x9cgassing potential.xe2x80x9d The gassing potential relates to the open circuit terminal voltage at or above which electrolyte begins to evaporate. In a typical VRLA battery, any generated gas is vented through a relief valve and cannot be replaced by adding water, as is commonly done with conventional/flooded batteries. Hence, gassing results in reduced battery capacity due to the reduced electrolyte.
Even at low charging currents, the potential exists for thermal runaway and, possibly, battery melt down. Thermal runaway may occur during charging when the rate of internal heat generation, resulting from the exothermic reaction at the negative plate due to oxygen recombination, exceeds the rate at which the generated heat can be dissipated. Undercharging, however, is not a satisfactory solution to such heat problems because undercharging a battery severely shortens the number of charge-discharge cycles that such a battery may experience before it fails.
It has been discovered that when a typical battery""s state of charge is less than 50 percent of its capacity, virtually all of the charging current supplied is consumed in charge reactions and no gas is produced. Thus, when charging such low charge state sealed batteries, any concerns for gassing are typically diminished. If, however, the battery charge state exceeds roughly fifty percent, the potential for gassing increases. In the latest stages of the charging process, for example, battery chargers often supply voltages at which some, or even most, of the input current is consumed in the gassing process.
Apart from gassing, there are other problems associated with charging lead acid batteries. A deeply discharged battery often demonstrates a very low initial charge acceptance. In other words, deeply discharged batteries resist charging. The problem typically worsens if the discharged battery is allowed to sit for a long period of time (e.g., weeks or months) in the deeply discharged state. A condition known as sulfation causes this charge resistance.
Sulfation occurs when sulfur from battery acid (sulfuric acid) deposits on the plates of a battery. Sulfation severity increases as a battery discharges. Sulfation adversely affects charge acceptance because the sulfation process causes electrolyte inefficiency due to the reduction of sulfur in the electrolyte.
When a battery has not been allowed to deeply discharge, the normal recharging process removes a sufficient amount of the sulfation film, thereby allowing a satisfactory recharge. If the sulfation problem is severe, it becomes impractical to recharge the battery. Moreover, severe sulfation or repeated sulfation can result in plate erosion, which, in turn, adversely affects battery performance.
As already indicated, a sulfated battery initially accepts very little charge current, even though the applied voltage may be quite substantial. As the charging process continues, the sulfate crystals break down, allowing an increased charge current and increased charge acceptance. Prior art battery chargers, however, often use automatic controllers that step down to stay below some voltage limit, and as such, do not properly step back up as the sulfated battery begins to accept charge. Further, prior art constant current battery chargers either provide too little charge current when the battery charge state is less than 50 percent, or provide too much current at higher charge states.
Further, when attempting to charge a battery, it may not be known whether the battery can be recovered. In other words, a weak battery may be subjected to an entire charging process before it is determined that the battery is not serviceable.
There is a need, therefore, for a battery charger that provides an optimal charging current profile with respect to the charge state of the battery, yet does not apply a voltage in excess of the gassing potential. Thus, there is a need for a voltage controlled battery charger that allows for maximum charging current without causing undesirable gassing or heating. There is further a need for a battery charger that provides for improved charge acceptance of sulfated batteries. There is another need for a battery charger that provides an indication of battery capacity during the charging process so that a bad or weak battery can be identified early in such process.
The charger and method of the invention has a number of advantages over the prior art including the provision of a battery charger that substantially reduces the likelihood of gassing during the charging process by maintaining the charging signal at a level sufficiently high to efficiently charge the battery, yet sufficiently low as to prevent gassing. The invention also provides for improved charge acceptance of sulfated batteries by attempting to desulfate such batteries before charging such batteries. The charger and method advantageously monitor battery characteristics during the charging process so that a bad or weak battery may be identified in an efficient manner and removed from service.
In an exemplary embodiment, the invention includes a method for charging a battery having battery terminals. The method comprises several steps including initiating a charging process. A charging signal is applied to the battery terminals. The battery terminal voltage associated with the battery is measured. A ripple voltage component of the measured battery terminal voltage is determined. The determined ripple voltage is compared to a ripple voltage limit having a first value. The ripple voltage limit is decreased from the first value to a second value if the determined ripple voltage component is less than the first value of the ripple voltage limit. The charging process is terminated if the determined ripple voltage is greater than the ripple voltage limit.
Another embodiment of the invention includes a method for charging a battery for a charging time. The battery has battery terminals. The method comprises several steps which include initiating a battery charging process. A charging signal is applied to the battery terminals. The battery terminal voltage associated with the battery is measured. A ripple voltage component of the measured battery terminal voltage is determined. The determined ripple voltage component is compared to a ripple voltage limit having a first value. The charging process is terminated if the determined ripple voltage component is greater than the ripple voltage limit. The ripple voltage limit is adjusted as a function of the charging time such that the ripple voltage limit decreases from the first value to a second value as the charging time increases.
Another exemplary embodiment of the invention includes a battery charger for charging a battery for a charging time. The battery includes battery terminals, an electrolyte, and has a gassing potential at which the electrolyte tends to vaporize. The battery charger comprises a controller that selectively provides a power control signal. The controller includes a monitoring function that monitors a voltage at the battery terminals. A power application circuit applies a charging signal to the battery terminals in response to the power control signal. The controller includes a measuring function that selectively measures a ripple voltage associated with the voltage monitored at the battery terminals. The controller includes a ripple voltage comparing function that compares the monitored ripple voltage to a ripple voltage limit having a first value. The controller includes a limit adjusting function that adjusts the ripple voltage limit as a function of the charging time such that the ripple voltage limit decreases from the first value to a second value as the charging time increases.
In yet another exemplary embodiment, the invention comprises a method of charging a battery. The battery includes battery terminals, an electrolyte, and a predetermined gassing potential at which the electrolyte tends to vaporize. The method comprises applying a charging signal to the battery terminals. A battery terminal voltage associated with the battery is measured. The measured battery terminal voltage is compared to a voltage threshold. The applied charging signal is adjusted as a function of the measured battery terminal voltage such that the measured battery terminal voltage substantially remains below the predetermined gassing potential.
The invention also includes a method of charging a battery that includes battery terminals, an electrolyte, and a predetermined gassing potential at which the electrolyte tends to vaporize. The method comprises applying an initial charging voltage to the battery terminals. A battery terminal voltage associated with the battery is measured at the battery terminals. The measured battery terminal voltage is compared to an upper voltage threshold. The measured battery terminal voltage is compared to a lower voltage threshold. The applied charging is increased if the measured battery terminal voltage is less than the lower voltage threshold. The applied charging voltage is decreased if the measured battery terminal voltage is above the upper voltage threshold. The applied charging voltage is maintained if the measured battery terminal voltage is above the lower voltage threshold and below the upper voltage threshold, such that the measured battery terminal voltage substantially remains below the predetermined gassing potential.
In still another embodiment, the invention comprises a battery charger for charging a battery. The battery includes battery terminals, an electrolyte, and a predetermined gassing potential at which the electrolyte tends to vaporize. The battery charger comprises a controller selectively providing a power control signal. The controller includes a monitoring function that monitors a voltage at the battery terminals. A power application circuit selectively applies a charging signal to the battery terminals in response to the power control signal. The controller includes an adjusting function that adjusts the power control signal in response to the voltage monitored at the battery terminals such that the power control signal is adjusted to substantially inhibit the power application circuit from applying the charging signal at a level above the predetermined gassing potential.
In yet another exemplary form, the invention comprises a method of charging a battery having battery terminals. The method comprises initiating a battery charging process. A charging signal is applied to the battery terminals. A battery terminal voltage associated with the battery is measured. A ripple voltage component of the measured battery terminal voltage is determined. The determined ripple voltage component is compared to a ripple voltage limit having a first value. The ripple voltage limit is adjusted from the first value to a second value as a function of the difference between the determined ripple voltage component and the ripple voltage limit. A determination is made as to whether the battery is sulfated. If the battery is sulfated, a desulfation process is performed. The desulfation process comprises applying a pulse charge voltage charging profile to the battery terminals. A charge acceptance charging profile is applied to the battery terminals upon completion of the application of the pulse charge voltage charging profile.
Still another exemplary embodiment of the present invention comprises a method of charging a battery comprising battery terminals, an electrolyte, and having a gassing potential at which the electrolyte tends to vaporize. The method comprises initiating a battery charging process. A charging signal is applied to the battery terminals. A battery terminal voltage associated with the battery is measured. A ripple voltage component of the measured battery terminal voltage is determined. The determined ripple voltage component is compared to a ripple voltage limit having a first value. The ripple voltage limit is adjusted from the first value to a second value as a function of a difference between the determined ripple voltage component and the ripple voltage limit. The measured battery terminal voltage is compared to a voltage threshold. A difference between the battery terminal voltage and the voltage threshold voltage is determined. The charging signal is adjusted as a function of the difference between the battery terminal voltage and the voltage threshold such that the measured battery terminal voltage substantially remains below the gassing potential.
In still another exemplary form, the invention comprises a method of charging a battery comprising battery terminals, an electrolyte, and having a gassing potential at which the electrolyte tends to vaporize. The method comprises initiating a battery charging process. A charging voltage is applied to the battery terminals. A battery terminal voltage associated with the battery is measured. A ripple voltage component of the measured battery terminal voltage is determined. The determined ripple voltage component is compared to a ripple voltage limit having a first value. The ripple voltage limit is adjusted from the first value to the second value as a function of the difference between the determined ripple voltage component and the ripple voltage limit. The measured battery terminal voltage is compared to an upper voltage threshold. The measured battery terminal voltage is compared to a lower voltage threshold. The applied charging voltage is increased if the measured battery terminal voltage is below the lower voltage threshold. The applied charging voltage is decreased if the measured battery terminal voltage is above the upper voltage threshold.
Alternatively, the invention may comprise various other methods, devices, and/or systems.
Other objects and features will be in part apparent and in part pointed out hereinafter.